The present invention relates generally to lifting devices, such as cranes or the like, and more particularly to a boom assembly comprised of a plurality of telescoping boom sections and corresponding interchangeable wear pads for reducing the friction during the relative telescoping movement.
Various types of telescoping booms or boom assemblies for use in cranes or other lifting devices are known in the art. Typically, the boom assembly includes a plurality of nested tubular sections, with each outer section having a larger cross-sectional size than the next-adjacent inner section to permit the desired relative telescoping movement. In the typical arrangement, the proximal end of the outermost boom section is pivotally mounted on a turntable, and the distal end of the innermost section carries one or more sheaves or equivalent structures for supporting the hoisting cable or the like. Friction-reducing structures, such as wear pads, shoes (also known as xe2x80x9cslippersxe2x80x9d), or like structures formed of a wear-resistant material with enhanced tribological characteristics are normally provided between the adjacent boom sections to provide support and reduce friction during the relative telescoping movement. A basic example of such an arrangement is shown in Kidde""s U.S. Pat. No. 5,829,606 to Erdmann.
In recent years, the general direction of progress in the art has been away from a conventional boom section formed having a square or rectangular cross-sectional profile, as shown in the ""606 patent, and towards a boom section having a cross-sectional profile that is at least partially rounded. As should be appreciated, a boom section with a partially rounded profile requires less material to manufacture and is thus lighter in weight. In addition to reducing weight and conserving material, it has been discovered that a boom section with a partially rounded profile is able to resist equal or greater bending stresses than the square or rectangular counterpart. Representative examples of proposals for modem telescoping boom assemblies including having partially rounded boom sections are found in Grove""s U.S. Pat. No. 6,108,985 to Paschke et al., Kidde""s U.S. Pat. No. 5,884,791 to Vohdin et al., and Liebherr""s European Published Patent Application 0 583 552. In particular, each of these references discloses a boom assembly including a plurality of telescoping boom sections, each having a rounded lower portion defined by a plurality of non-equidistant, non-equiangular breaks.
Despite the advantages generally afforded by the partially rounded profiles formed by breaks of the type disclosed in these references, a significant, but heretofore unrecognized problem arises with regard to the corresponding wear shoes or pads used to reduce the friction between the lower front ends of the boom sections during telescoping movement. In particular, since the angle forming each break and the distance between adjacent breaks is substantially different, a plurality of different sizes and shapes of wear pads are required for engaging one or more of the different length segments (xe2x80x9cflatsxe2x80x9d) created. This problem is further compounded by the fact that different sizes of wear pads are required for supporting the various different sizes of nesting boom sections. For example, in the boom assembly disclosed in the Grove ""985 patent, at least three different types of wear xe2x80x9cslippersxe2x80x9d are required for positioning between the lower front ends of each adjacent pair of telescoping boom sections. In the case where three telescoping boom sections form the boom assembly, at least six different types of wear slippers must be kept on hand or in inventory; for four boom sections, at least nine different types of wear slippers are required; and telescoping boom assemblies formed of five boom sections require at least twelve different types of wear slippers.
As should be appreciated, the requirement for having multiple types of non-interchangeable wear pads is disadvantageous, since it increases the cost from both the manufacturing and production standpoint. An associated consequence is the need for the crane operator to keep the various different types of wear pads on hand at the job site, in case there is an immediate need for replacement. Even in the case where the wear pads are inspected between jobs and replaced if excessive wear is observed, the manufacturer of the crane must still keep a ready supply of the different types of replacement wear pads available throughout the service life of the crane or other lifting device using such a boom assembly. This not only increases the amount of inventory and the associated expense that must be borne by the crane manufacturer (and ultimately passed on to the crane operator), but it also creates deleterious inventory management and control problems. The net result is an increase in the overall manufacturing and operational cost of the crane or other lifting device having a boom assembly with several different types of non-identical wear pads or shoes for supporting the lower front end of each adjacent boom section during telescoping movement.
Accordingly, a need is identified for an improved boom assembly and a corresponding arrangement of friction or wear-reducing structures that address and overcome the foregoing limitations of the prior art arrangements. In particular, the telescoping sections forming the boom assembly would have the desirable partially rounded profile for reduced weight, enhanced stiffness and added strength, but would be adapted for receiving a plurality of interchangeable wear pads along the rounded portion. As a result, the need for keeping different types of wear pads on hand for supporting each boom section and by engaging the rounded portion would be eliminated, and an improved boom assembly would be provided, especially in terms of reduced manufacturing and operational expense.
In accordance with a first aspect of the invention, a boom assembly for a crane or other lifting device is provided. The boom assembly is comprised of at least two adjacent telescoping boom sections. A first of the telescoping boom sections includes a rounded portion formed by a plurality of substantially equiangular breaks. A plurality of substantially identical wear pads are also provided as part of the assembly for reducing friction between the adjacent boom sections during telescoping movement. In use, each wear pad is positioned adjacent to one of the breaks relative to the first boom section and supported by a matching second, adjacent telescoping boom section. Hence, the wear pads not only reduce friction in the desired manner, but also are freely interchangeable. Advantageously, this eliminates the need for keeping a plurality of different types of non-identical replacement wear pads on hand such as for positioning between the lower front ends of adjacent boom sections, either at the job site or in the inventory kept by the manufacturer at a remote location.
In one embodiment, each of the plurality of breaks formed in each side of the lower portion of the first boom section are equidistant from each other. Additionally, at least a portion of the outer surface of the first boom section on either side of each break is substantially flat. To ensure that intimate contact is established with the corresponding boom section, including the flat segments between the adjacent breaks, each substantially identical wear pad is provided with a contoured engagement surface. This contoured engagement surface is preferably substantially V-shaped to match the shape of the corresponding outer surface of the boom section. However, the use of other equivalent shapes or contours capable of achieving the desired friction-reducing engagement is also possible.
Normally, the boom is formed of three or more nested telescoping boom sections, each identical in shape and increasing in cross-sectional size from the innermost to the outermost boom section. In the case of an innermost boom section having a smaller cross-sectional profile than any of the outer boom sections, each identical wear pad is preferably sized such that it may be positioned adjacent to an adjoining wear pad to create a substantially continuous wear surface. For the outer boom sections, each of which is slightly larger in cross-section than the next-adjacent inner boom section to permit telescoping, a plurality of spacers are provided for filling any gap between adjacent identical wear pads positioned at the breaks (each of which may also be equidistant from each other along each side of the boom section, but of course are farther apart than the equidistant breaks on the next adjacent inner boom section). As should be appreciated, the spacers also keep the wear pads positioned at the breaks by preventing shifting during the telescoping of the adjacent boom section.
A retainer is also preferably provided on each outer boom section for holding the wear pads and/or spacers in place during the relative telescoping movement of the next-adjacent inner boom section. As a result of using identical wear pads, the retainer may have a relatively simple construction, as compared to past arrangements where separate retainer structures are required for accommodating each different type of wear pad.
Friction-reducing support structures, such as L-shaped wear shoes, may also be provided on each outer boom section for engaging an upper surface of the adjacent nested boom section. These upper wear shoes may be removed from the mounted position to allow for the lifting of the corresponding next-adjacent boom section. This lifting creates a clearance or space for gaining access to the wear pads and/or spacers without the need for completely disassembling the boom assembly. A mechanism may also be provided for permitting the selective adjustment of the position of the upper wear shoes relative to the adjacent upper portion of the corresponding boom section. This feature not only allows for the degree of frictional engagement with the wear pads to be selectively controlled, but also advantageously allows a worker to fine tune the alignment of the adjacent boom section.
In accordance with a second aspect of the invention, a crane including an improved apparatus for assisting in lifting or moving a load is provided. The apparatus comprises a telescoping boom assembly including at least two adjacent telescoping boom sections. A first of the boom sections includes a rounded portion having a plurality of substantially equiangular breaks. A plurality of substantially identical wear pads are provided for reducing friction between the adjacent boom sections during relative telescoping movement. In particular, each wear pad is positioned adjacent to one of said breaks of the first boom section and supported by a second, adjacent telescoping boom section. In addition to reducing friction, the identical wear pads are fully interchangeable, which advantageously eliminates the need for keeping a plurality of substantially different wear pads on hand.
In accordance with a third aspect of the invention, a friction reducing structure is provided for use in a crane or lifting device having a boom assembly comprised of first and second adjacent telescoping boom sections, with at least the first boom section having a rounded portion defined by a plurality of substantially equiangular breaks. The friction reducing structure comprises a plurality of substantially identical wear pads having V-shaped engagement surfaces, each for positioning adjacent to one of the equiangular breaks of the first boom section and supported by the second, adjacent telescoping boom section. The substantially identical wear pads or shoes not only reduce friction between the adjacent boom sections, but are interchangeable to eliminate the need for keeping a plurality of substantially different sizes or shapes of wear pads for positioning between the lower front ends of adjacent boom sections on hand.
In accordance with a fourth aspect of the invention, a method for reducing friction in a boom assembly including at least two adjacent telescoping sections is provided. The method comprises placing a plurality of substantially identical wear pads adjacent to a corresponding plurality of substantially equiangular breaks forming a rounded portion of a first of the two adjacent telescoping boom sections. As a result of using substantially identical wear pads, the costs and other difficulties associated with controlling a large inventory of non-identical wear pads are substantially reduced while the desirable friction reduction between the adjacent boom sections during telescoping is advantageously achieved in the desired manner.
To place the identical wear pads in the operative position, raising the first boom section relative to a second adjacent boom section may be required, which as described above may involve removing any upper wear shoes or other friction reducing structures present. This creates a clearance or space between the adjacent boom sections for inserting or removing each of the substantially identical wear pads. In the case where the wear pads positioned at the breaks are not adjoining or contiguous, such as on outer boom sections, the step of placing may further include providing a spacer in any gap or space between the substantially identical wear pads positioned at adjacent breaks. The spacers substantially prevent the wear pads from shifting relative to each other.
In one embodiment of the method, a plurality of rounded boom sections are provided, each having an identical number of breaks. However, because each outer boom section is larger than the next-adjacent inner boom section, the corresponding breaks are equidistant but are spaced farther apart from each other. In this case, the step of placing further includes: (1) providing a first spacer having a first width dimension between the wear pads adjacent to a second boom section having a plurality of equidistant breaks that are closer together than a next-adjacent third boom section; and (2) providing a second spacer having a second width dimension between the wear pads adjacent to a fourth boom section having a plurality of equidistant breaks that are spaced farther apart than the breaks on the third boom section. Using this arrangement, the wear pads remain interchangeable among the different sizes of telescoping boom sections, with only the dimensions of the spacers changing between the adjacent outer boom sections.